The team was able to charge an electronic device smaller than a grain of rice at depths of up to 10cm

Breakthrough in wireless charging of medical implants

A new method for wirelessly charging tiny implants deep within the body could revolutionise the treatment of a host of diseases.

While advances in integrated circuits have enabled the miniaturisation of electronic medical implants, the miniaturisation of their power sources has failed to keep pace meaning that devices come with bulky battery packs.

Previous studies have investigated using near-field wireless charging to power such devices, but the inability of the electromagnetic waves to penetrate deep into the body prevents it from being used for implants that are deeper than about 1cm – ruling out there use for treating heart and brain conditions among others.

But by taking advantage of what they call ‘mid-field’ wireless charging, researchers from Stanford University have managed to power a 2mm implant on the surface of a rabbit’s heart that enabled regulation of its cardiac rhythm at a depth of 4.5cm.

“Integrated circuit technology can make devices so incredibly small already and they can do an amazing number of things. The only thing that holds them back is the power source, so what we’ve done with this work is replace the power source,” said PhD student John Ho, the lead author of a study published in the Proceedings of the National Academy of Sciences detailing the research.

The researchers, working under assistant professor of electrical engineering Ada Poon, created a new kind of antennae – a 6x6cm patterned metal plate – that generates a special type of near-field wave that changes characteristics when it moves from air to skin enabling it to propagate through tissue, like sound waves through a railway track.

So far the researchers have tested the method to a depth of up to 10cm using porcine tissue – pig flesh. The new power transfer method uses roughly the same power as a cell phone with exposure levels well below the safety threshold for humans.

While a direct link to the power source is currently needed to power the microimplant, Ho and colleagues are investigating adding tiny batteries to the devices that can then be recharged using the midfield wireless power transfer system – a feat not possible with current technologies.

The implants could have a host of applications in the emerging field of ‘electroceuticals’ – which uses electrical devices to treat disease rather than drug therapies – from regulation of cardiac rhythms, deep brain stimulation for the treatment of depression, or even real-time monitoring of chronic disease.

“The remarkable thing about integrated circuit technology is that without making the device any larger – the current device is quite large by integrated circuit standards – we can add extra functionality to the chip. The bottleneck is power,” said Ho.

“We could put a camera on it to look inside your body; people have done things like sensors for neural recording; you could even look at locomotion.”

Other researchers looking at solving the issue of powering microimplants have turned to energy harvesting devices that convert ambient energy into a power source for the circuits, but Ho believes his team’s method is superior.

“Energy harvesting works only in very specific parts of the body. You can do it in the heart or the lungs but as far as the brain’s concerned it wouldn’t work,” he said.

“Current energy harvesting devices out there tend to be rather large because of the low efficiency in power conversion processes. The advantage with wireless charging is you are going from electromagnetic to electromagnetic so you there’s no loss there.”

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